Rename the shared contract module danos -> system; QEMU logs to /var/log/system

The shared kernel<->user ABI contract (BootInformation, the SystemCall numbers,
DeviceDescriptor, page_size, ...) is now the `system` module at
system/system.zig, following the convention that a directory's root file takes
the directory's name.

One overlap to note: the runtime's syscall wrappers are already `runtime.system`,
so the single file that uses both the contract and those wrappers
(library/runtime/heap.zig) aliases the wrappers locally as `system_calls`. The
two are distinct (top-level `system` vs `runtime.system`); everywhere else the
contract is just `system`.

Also: the QEMU run's serial capture now lands in the FHS log location,
zig-out/var/log/system/serial0-<timestamp>.log — a stand-in for the kernel's own
logging system, which will eventually write there itself.

Suite 35/35 plus host tests green.

docs/logging.md

@@ -58,7 +58,7 @@ screen. `console.write` is a no-op when the firmware gave us no framebuffer.
 A framebuffer is not guaranteed — a headless server exposes no UEFI Graphics Output
 Protocol. That used to be *fatal* (the loader failed the boot). Now the loader hands
 over a "no framebuffer" descriptor (`base == 0`) rather than failing, and
-`Framebuffer.present()` (in `system/danos.zig`) gates every on-screen path. A headless,
+`Framebuffer.present()` (in `system/system.zig`) gates every on-screen path. A headless,
 serial-less machine boots and runs correctly — it just goes quiet.
 
 ## Last-resort channels (no text output at all)